A color display may be obtained by the appropriate overlay of two or more light valve images. It is preferred, however, to employ an optical system in which multiple images are formed on a single light valve and to overlay the images with a single projection lens. This technique facilitates the focusing and alignment of the images. Such an optical system is described by Dewey in the IBM Technical Disclosure Bulletin Vol 21, No. 3 August 1978, pages 1201 and 1202. This system uses an off axis projection in which light of one color, for example, red, is produced by a condenser system and the projection light of a different color, for example, green, is provided by a second condenser system. The two images are coprojected by a lens onto the screen with the use of parallel pairs of folding mirrors.
There are two major problems with this optical system, both of which are solved by the present invention. The first problem relates to the distortion of the projection lens. FIG. 1 shows the field of view of the projection lens along with the position of two images 20A and 20B written on the light valve that are overlaid by the projection system. The optical system attempts to overlay the following pairs of points: (1,2'), (2,1'), (3,4') and (4,3'). However, distortion in the projection lens prevents these points from being overlaid exactly and the result is misconvergence of the two images. Consequently it would be necessary to employ an expensive projection lens having very low distortion in order to achieve adequate color convergence in the display.
The second problem is that because of the off-axis projection system, the two colored beams converge onto the projection screen at equal but opposite angles. All projection screens are directional to some extent and rear-projection screens are strongly directional. That is, they are not perfect diffusers and the brightness of a given point on the screen depends on the position of the viewer. Thus, when two colored beams are incident at different angles onto the screen, the resultant color of a point on the screen depends on the position of the viewer. For example, with red and green beams incident on the screen, a point will appear yellow from one position, but may appear orange from another viewpoint and yellow-green from yet another. Moreover, from a fixed viewpoint, different points of the screen will have different colors. The result is a display that has non-uniform color and in which the color changes as the viewer moves. Such a system is not desirable. Front projection screens may be made with very little directionality and in this case the color uniformity may be adequate. However, such screens are usually used for group viewing with long projection distances, and hence the final two folding mirrors which direct the two colored beams onto the screen would have to be widely separated. This would be impractical for general use. This type of optical system applies to either a reflective light valve or a transmissive light valve.
The patent to Hareng et al, U.S. Pat. No. 4,150,396, describes two embodiments of a projection system that creates a color display from a transmissive, laser-addressed liquid crystal light valve (or valves). As in the present invention, both systems employ a single projection lens, however, they may only be used with transmissive light-valves. Moreover, the first embodiment is impractical in that the optical path length is different for the three colored images. Thus, it is impossible to focus all three on a single screen and overlay the images. In the second embodiment, which employs three light valves, the path length is the same for all three images, but the system can only be used with a projection lens having a very narrow field of view. This implies a long projection distance to achieve a screen image of adequate size.
The patent to Kidwell and Tsai, U.S. Pat. No. 4,345,258, describes a color display system in which a pair of transmissive laser-addressed liquid crystal light valves are co-projected onto a screen using a single projection lens and a dichroic mirror. This system does not use a single light valve for the two images, and is not applicable to a reflective light valve.